The present invention relates to a circuit for playing back three-line sequential color television signals, particularly for use in a video disc player.
It is already known in the art to record a color television signal on a recording device having a narrow bandwidth, typically about 3 MHz, by recording the three color component signals representing the primary colors in line-by-line alternance and, during playback, to make signals representing the three color components simultaneously available by employing a series connection of two line delay lines, as disclosed in German Pat. No. 1,261,876 and corresponding U.S. Pat. No. 3,560,635, issued to Walter Bruch.
In this connection, it is also known to effect this sequential recording of the primary color signals only in the lower frequency band of about 0-0.5 MHz and to record during each picture line period a luminance signal in the upper frequency band of about 0.5-3 MHz. During playback, the signals in these two frequency bands coming from the recording device are first separated by a frequency filter and directed into two respective channels. The first channel permits only signal components in the lower frequency range to pass and processes the sequential color signals. The second channel permits only the signal components in the upper frequency band to pass, i.e. it furnishes the high frequencies of the luminance signal which are subsequently recombined with the processed signals in the lower frequency band.
In such a circuit, the signal components in the lower frequency band always pass only through the first channel. Thus all interferences and errors occurring in the first channel, e.g. distortions from modulators, delay lines and filters, become fully effective even in the case of a monochrome picture signal.
It is known to reduce these errors, as disclosed in German Pat. No. 2,207,021 and corresponding U.S. Pat. No. 3,808,359 issued to Werner Scholz, by transmitting signal components in the first frequency band also in the second channel and adding a difference signal in this channel, which difference signal is obtained by a special matrixing of the signals from the first channel. Preferably, the signals are combined at the outputs of a series arrangement of delay lines, each effecting a delay of one horizontal scanning line period, in a certain amplitude ratio to form the difference signal.
A playback circuit for such three-line sequential color television signals is also disclosed in German Offenlegungsschrift [Laid Open Application] No. 24 46 376, in which a third delay line having the same delay period is disposed in series with the series circuit of the two above-mentioned delay lines. The difference signal is then formed by utilizing the input signal to the series circuit and the signals at the outputs of the three delay lines.
This playback circuit is particularly suitable for the playback of a signal which is subject to predistortion during recording. This predistortion, which is also described in German Offenlegungsschrift No. 24 46 376, serves the purpose of reducing the errors occuring during playback upon the occurrence of substantial transitions in the vertical direction, i.e. at the horizontal edges, as a result of the trisequential operation. The predistortion substantially involves averaging of a plurality of lines of the sequential signals.
The known playback circuit having a total of three delay lines is of relatively complicated structure because individual signals which have different amplitudes have to be combined to form the luminance difference signal. This special requirement for different amplitudes also makes matching of the components of this playback circuit relatively difficult.